Filter device and duplexer

ABSTRACT

In a filter device, a transversal elastic wave filter, which defines a delay element, is connected in parallel with a band pass filter. The transversal elastic wave filter has the same amplitude characteristic as and the opposite phase to the band pass filter at a desired frequency inside an attenuation range of the band pass filter. When a wavelength determined by an electrode finger period of IDTs and is denoted by λ, the distance between the first IDT and the second IDT of the elastic wave filter is about 12λ or less.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a filter device in which a delayelement is connected in parallel with a band pass filter and to aduplexer that includes the filter device.

2. Description of the Related Art

In the related art, a variety of band pass filters are used in mobilecommunication devices such as cellular phones. In cellular phones and soforth, the frequency width between the passbands of a plurality ofchannels has been becoming smaller. Therefore, an increase in theattenuation in the vicinities of the passbands has been stronglydemanded.

In Japanese Unexamined Patent Application Publication No. 62-261211, afilter device is disclosed in which a delay element is connected inparallel with a main filter. The delay element has a characteristic thatit has substantially the same amplitude characteristic but a phase thatdiffers by (2n−1)π (n is a positive integer) at a desired frequencyinside an attenuation range of the main filter. Therefore, direct wavesat the desired frequency cancel each other out and attenuation can beincreased at that frequency.

In Japanese Unexamined Patent Application Publication No. 62-261211, thedelay element is formed by a surface acoustic wave (SAW) filter of atransversal type, for example.

However, in the filter device described in Japanese Unexamined PatentApplication Publication No. 62-261211, there is a problem in that thefrequency range of desired frequencies at which attenuation is desiredto be increased is narrow.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide a filter devicethat is capable of widening a frequency range across which attenuationis desired to be increased outside of a passband and to provide aduplexer that includes the filter device.

A filter device according to a preferred embodiment of the presentinvention includes a band pass filter; and a delay element that isconnected in parallel with the band pass filter and has a same amplitudecharacteristic as and an opposite phase of the band pass filter at adesired frequency inside an attenuation range of the band pass filter.The delay element preferably includes a transversal elastic wave filterincluding a first IDT and a second IDT, and a distance between the firstIDT and the second IDT is 12λ or less when a wavelength determined by anelectrode finger period of the IDT is denoted by λ.

In a certain specific aspect of the filter device according to variouspreferred embodiments of the present invention, the transversal elasticwave filter is a transversal elastic wave filter including a slanted, orinclined finger interdigital transducer (IDT). In this case, thefrequency range across which it is desired to increase attenuation iswidened even more.

In another specific aspect of the filter device according to variouspreferred embodiments of the present invention, the delay elementpreferably includes a plurality of transversal elastic wave filters andan electrode finger pitch of at least one of the transversal elasticwave filters is different from an electrode finger pitch of theremaining transversal elastic wave filter. In this case, it is possibleto increase the number of frequency bands across which attenuation isincreased and it is possible to further increase the frequency rangeacross which it is desired to increase attenuation.

In another specific aspect of the filter device according to variouspreferred embodiments of the present invention, the distance between thefirst IDT and the second IDT is preferably about 6λ or less. In thiscase, the frequency range across which it is desired to increaseattenuation is widened even more.

A duplexer according to another preferred embodiment of the presentinvention, which includes a first terminal that is connected to anantenna, a transmission terminal and a reception terminal, includes afirst filter unit that is connected between the first terminal and thetransmission terminal or the reception terminal, and includes any one ofthe filter devices according to a preferred embodiment of the presentinvention; and a second filter unit that is connected between the firstterminal and the reception terminal or the transmission terminal and hasa different passband to the first filter unit.

According to a filter device of a preferred embodiment of the presentinvention, the distance between IDTs in a delay element including atransversal elastic wave filter is preferably about 12λ or less andtherefore attenuation at a desired frequency inside an attenuation rangeis increased and the frequency range across which it is possible toincrease attenuation is effectively widened.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic block diagram illustrating a duplexer accordingto a first preferred embodiment of the present invention, and FIG. 1B isa schematic plan view of a transversal elastic wave filter used in thefirst preferred embodiment of the present invention.

FIG. 2 illustrates an amplitude characteristic of the transversalelastic wave filter used in the first preferred embodiment of thepresent invention and an amplitude characteristic of a capacitance of aband pass filter.

FIG. 3 illustrates a phase characteristic of the transversal elasticwave filter used in the first preferred embodiment of the presentinvention and a phase characteristic of the capacitance of the band passfilter.

FIG. 4 illustrates a transmission characteristic and a receptioncharacteristic of the duplexer of the first preferred embodiment of thepresent invention and illustrates a transmission characteristic and areception characteristic of a duplexer of a comparative example in whicha delay element is not connected.

FIG. 5 illustrates the transmission characteristic of the duplexer ofthe first preferred embodiment of the present invention and thetransmission characteristic of the duplexer of the comparative example.

FIG. 6 illustrates the reception characteristic of the duplexer of thefirst preferred embodiment of the present invention and the receptioncharacteristic of the duplexer of the comparative example.

FIG. 7 illustrates transmission waveforms and reception waveforms whenthe distance between IDTs of the transversal elastic wave filter in thefirst preferred embodiment of the present invention is preferably set tobe approximately 0.5λ, 5.6λ and 10.7λ.

FIG. 8 illustrates transmission waveforms when the distance between IDTsof the transversal elastic wave filter in the first preferred embodimentof the present invention is preferably set to be approximately 0.5λ,5.6λ and 10.7λ.

FIG. 9 illustrates reception waveforms when the distance between IDTs ofthe transversal elastic wave filter in the first preferred embodiment ofthe present invention is preferably set to be approximately 0.5λ, 5.6λand 10.7λ.

FIG. 10 illustrates the relationship between the distance between IDTsand a frequency range across which out-of-band attenuation is increased.

FIG. 11 is a plan view schematically illustrating the structure of atransversal elastic wave filter included in a second preferredembodiment of the present invention.

FIG. 12 illustrates an amplitude characteristic of the transversalelastic wave filter included in the first preferred embodiment of thepresent invention and an amplitude characteristic of the transversalelastic wave filter included in the second preferred embodiment of thepresent invention.

FIG. 13 illustrates a phase characteristic of the transversal elasticwave filter included in the first preferred embodiment of the presentinvention and a phase characteristic of the transversal elastic wavefilter included in the second preferred embodiment of the presentinvention.

FIG. 14 illustrates an attenuation-frequency characteristic of atransmission filter of the duplexer of the second preferred embodimentof the present invention and an attenuation-frequency characteristic ofa comparative example in which a transversal elastic wave filter is notconnected.

FIG. 15 is an enlarged view of an important portion of FIG. 14.

FIG. 16 illustrates at an enlarged scale transmission waveforms of thesecond preferred embodiment of the present invention when the distancebetween IDTs is about 1.3λ and of third and fourth preferred embodimentsof the present invention in which a slanted structure is not providedand the distance between IDTs is about 1.3λ or about 10.8λ.

FIG. 17 illustrates at an enlarged scale transmission waveforms of thesecond preferred embodiment of the present invention when the distancebetween IDTs is about 1.3λ and of the third and fourth preferredembodiments of the present invention in which a slanted structure is notprovided and the distance between IDTs is about 1.3λ or about 10.8λ.

FIG. 18 illustrates amplitude characteristics of a transversal elasticwave filter and a capacitance of a band pass filter.

FIG. 19 illustrates phase characteristics of a transversal elastic wavefilter and a capacitance of a band pass filter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, the present invention will be made clearer by describingspecific preferred embodiments of the present invention while referringto the drawings.

FIG. 18 and FIG. 19 are drawings for describing a problem with a filterdevice of the related art. FIG. 18 illustrates amplitude characteristicsfor a case where a band pass filter and a transversal surface acousticwave filter, in which the distance between IDTs is 10.6λ, are connectedin parallel with each other. FIG. 19 illustrates the phasecharacteristics. The alternate long and short dash lines in FIG. 18 andFIG. 19 respectively represent the attenuation and phase of acapacitance of the band pass filter.

As is clear from FIG. 18 and FIG. 19, the frequency interval acrosswhich the phase characteristic of the band pass filter, that is, thecapacitance of the band pass filter and the phase characteristic of thetransversal elastic wave filter have opposite phases to each other isvery narrow. In FIG. 19, the frequency at which the phase of thecapacitance is around 90° and the phase of the transversal elastic wavefilter becomes the opposite phase of −90° is 710 MHz. At this time, thefrequency range across which the phase of the elastic wave filter is inthe range of −90°±30° is 706 MHz to 714 MHz. That is, the frequencyrange is only 8 MHz. Here, transmission of a signal occurs due to astray capacitance in an attenuation range of the band pass filter. If itis considered that the wavelength of a high-frequency signal thatpropagates due to a stray capacitance is several m to several cm at afrequency of several hundred MHz to several GHz and that the propagationdistance of the high-frequency signal that propagates due to the straycapacitance is several μm to several mm, the wavelength of thehigh-frequency signal is sufficiently larger than the propagationdistance of the signal that propagates due to the stray capacitance.Therefore, the change in phase with a change in frequency is very small.In contrast, the speed of sound in a transversal surface acoustic wavefilter is as slow at around 3000 to 4000 m/s. If it is considered thatthe wavelength of a high-frequency signal that propagates as a surfaceacoustic wave is several μm at a frequency of several hundred MHz toseveral GHz and the propagation distance of the surface acoustic wave isequal to or less than the device size at most, then the propagationdistance of the surface acoustic wave is several μm to several mm.Therefore, it cannot be said that the wavelength of the high-frequencysignal is sufficiently larger than the propagation distance. Therefore,the change in phase with a change in frequency is large as describedabove. If the distance between the IDTs is increased, the change inphase with respect to frequency will become even larger. Therefore, ifthe distance between the IDTs is increased, it will be difficult toimprove attenuation across a wide frequency range.

In contrast, in the preferred embodiments and modifications thereof ofthe present invention to be described below, it is possible to widen afrequency range across which attenuation is improved.

FIG. 1A is a schematic block diagram illustrating a duplexer accordingto a first preferred embodiment of the present invention. A duplexer 1includes an antenna terminal 3 that is connected to an antenna 2. Acommon connection terminal 4 is connected to the antenna terminal 3. Amatching inductor 5 is connected between the common connection terminal4 and a ground potential. A transmission filter, which includes a filterdevice 7 of a preferred embodiment of the present invention, and areception filter 8 are connected to the common connection terminal 4.The filter device 7 includes a signal terminal 6 and a transmissionterminal 9. A transmission signal is input from the transmissionterminal 9.

The filter device 7 includes a band pass filter 10, which includes aladder filter including a plurality of series-arm resonators S1 to S5and a plurality of parallel-arm resonators P1 to P3. A transversalsurface acoustic wave filter is connected in parallel with the band passfilter 10 and serves as a delay element.

The reception filter 8 is connected to the common connection terminal 4.The reception filter 8 has reception terminals 12 and 13. The receptionfilter 8 is composed of a suitable band pass filter circuit such as alongitudinally coupled resonator-type elastic wave filter.

One of the unique characteristics of this preferred embodiment is thatthe distance between IDTs of the transversal surface acoustic wavefilter 11 is small at about 12λ or less, for example, and the surfaceacoustic wave filter 11 preferably has the same amplitude as and theopposite phase to the band pass filter 10 at a desired frequency outsideof the band of the band pass filter 10. Therefore, out-of-bandattenuation is increased at the desired frequency and it is possible towiden a frequency range across which out-of-band attenuation is large.

As illustrated in FIG. 1B, the transversal surface acoustic wave filter11 has a structure in which a first IDT 15 and a second IDT 16 areprovided on a piezoelectric substrate 14. In the first preferredembodiment, the distance between IDTs, which is determined by a distancebetween the centers of electrode fingers, between the first IDT 15 andthe second IDT is preferably small at about 12λ or less, for example.Therefore, the frequency range in which there is the relationship forthe same amplitude and opposite phase is widened. This will be describedwith reference to FIG. 2 to FIG. 9.

The above-mentioned ladder filter, which includes surface acoustic waveresonators, was used in the band pass filter 10 when forming theduplexer 1. A surface acoustic wave filter was also used for thereception filter 8. A piezoelectric substrate composed of 42° Y cut Xpropagation LiTaO₃ was used as the piezoelectric substrate 14. The bandpass filter 10 and the reception filter 8 having the above-describedcircuit configurations were provided on the piezoelectric substrate 14.

The first IDT 15 and the second IDT 16 were provided on thepiezoelectric substrate 14 to define the surface acoustic wave filter11.

The surface acoustic wave filter 11 had the following configuration.

The number of pairs of electrode fingers of the first IDT 15 was 3, thenumber of pairs of electrode fingers of the second IDT 16 was 15, theintersecting width in the first IDT 15 and the second IDT 16 was about60 μm and the wavelength λ determined by the electrode finger period wasabout 5.5 μm, for example. The distance between the first IDT 15 and thesecond IDT 16 was about 0.5λ, for example.

The solid line in FIG. 2 represents an amplitude characteristic of thesurface acoustic wave filter 11 and the solid line in FIG. 3 representsa phase characteristic of the surface acoustic wave filter 11. Inaddition, the alternate long and short dash lines in FIG. 2 and FIG. 3respectively represent the amplitude characteristic and the phasecharacteristic of a capacitance of about 0.001 pF representing the bandpass filter 10.

As is clear from comparing FIG. 2 and FIG. 3 and FIG. 18 and FIG. 19,the frequency range across which the phase characteristic of thecapacitance of the band pass filter 10 and the phase characteristic ofthe surface acoustic wave filter 11 have opposite phases to each otheris wider than in the case of the amplitude characteristics and the phasecharacteristics illustrated in FIG. 18 and FIG. 19. For example, in FIG.3, the frequency at which the surface acoustic wave filter 11 has theopposite phase to the capacitance of the band pass filter 10, that is,has a phase of about −90° is about 744 MHz. Then, for example, thefrequencies at which the phase of the surface acoustic wave filter 11 isabout −90°±30° are about 736 MHz to about 753 MHz and therefore thefrequency range is about 17 MHz. In other words, the phase of thesurface acoustic wave filter 11 is about −90°±30° across a frequencyrange that is over two times the width of the frequency range of about 8MHz in the case of the configuration illustrated in FIG. 18. Therefore,it is clear that the frequency range across which the amplitudes are thesame and the phases are opposite are widened.

The reason for this is as follows. In this preferred embodiment, theinterval between the first IDT 15 and the second IDT 16 preferably isset to be small at about 0.5λ, for example. Consequently, the change inphase with frequency of the surface acoustic wave filter 11 is small.Therefore, the phase characteristic of the band pass filter 10 and thephase characteristic of the surface acoustic wave filter 11 aremaintained in a state of having close to opposite phases across a widefrequency range. Therefore, according to this preferred embodiment,attenuation is increased across a wide frequency range.

In addition, in various preferred embodiments of the present invention,it is assumed that the meaning of “same amplitudes” includes not onlythe case where the amplitude of the band pass filter and the amplitudeof the elastic wave filter are the same as each other, but also includesa case where an amplitude X of the elastic wave filter and an amplitudeY of the band pass filter when the band pass filter is connected to theelastic wave filter are within a range of about 10 log₁₀ X/Y=−5 dB to +3dB for the amplitudes of the signals of the two filters, for example. Asdescribed in Japanese Unexamined Patent Application Publication No.62-261211, it is possible to increase the attenuation in a state wherethe amplitudes are the same as provided by various preferred embodimentsof the present invention. Therefore, not limited to the case in whichthe amplitude of a signal of the band pass filter 10 and the amplitudeof a signal of the surface acoustic wave filter 11 are equal to eachother, it is sufficient that the amplitudes of the signals be within thesame amplitude range described above which includes the case where theamplitudes of the signals are equal to each other.

In addition, “opposite phases” is not limited to just the case where thephase of the band pass filter 10 and the phase of the surface acousticwave filter 11 are completely opposite to each other. That is, it issufficient that the difference between the phase of the band pass filter10 and the phase of the surface acoustic wave filter 11 be with a rangeof about 180°±30°, for example.

The broken line A and the broken line B in FIG. 4 respectively representa transmission waveform Tx and a reception waveform Rx of the duplexer1. In addition, the solid line and the two-dot chain line respectivelyrepresent a transmission waveform and a reception waveform of a duplexerin which the surface acoustic wave filter 11 is not connected. FIG. 5illustrates the transmission waveform of the duplexer and thetransmission waveform of the comparative example extracted from FIG. 4.FIG. 6 illustrates the isolation characteristic of the duplexer and theisolation characteristic of the comparative example extracted from FIG.4. In FIG. 5 and FIG. 6, the broken lines A and B illustrate the resultsof a preferred embodiment of the present invention and the solid linesillustrate the results of the comparative example.

According to the present preferred embodiment of the present invention,it is clear that attenuation of the transmission filter is improved inthe reception band Fx in FIG. 5 and FIG. 6. That is, it is clear that anincrease in attenuation of around 8 dB is achieved in the range of about746 MHz to about 756 MHz, which is the reception band Fx, in thetransmission waveform represented by the broken line A, compared withthe transmission waveform represented by the solid line.

In addition, it is clear from FIG. 6 that also in the isolationwaveform, an increase in isolation attenuation in the reception band Fxis achieved if the reception waveform of the present preferredembodiment represented by the broken line B and the reception waveformof the comparative example represented by the solid line are compared.Specifically, it is clear that an increase of around 10 dB is achievedin the isolation attenuation illustrated in FIG. 6 at a frequency ofabout 746 MHz, which is at the low side of the reception band Fx.

As described above, in this preferred embodiment, since the inter-IDTdistance between the first IDT and the second IDT preferably is narrowedto about 0.5λ, attenuation is effectively increased in the receptionband Fx.

FIGS. 7 to 9 illustrate the characteristics when the distance betweenthe IDTs is about 0.5λ as described above in this preferred embodiment,and when it is about 5.6λ and about 10.7λ, for example. In FIG. 7, thetransmission waveforms Tx and the reception waveforms Rx are superposedon one another and FIG. 8 illustrates the transmission waveforms andFIG. 9 illustrates the isolation waveforms.

As is clear from FIG. 8, attenuation is more effectively increased inthe reception band Fx when the distance between IDTs is about 0.5λ, forexample. In addition, when the distance between IDTs is about 10.7λ, thefrequency range across which the attenuation in the reception band Fx isimproved by about 3 dB or more is about 746 MHz to about 748 MHz, whichis about 2 MHz, for example. When the distance between IDTs is about0.5λ, the frequency range across which the attenuation in the receptionband Fx is improved by about 3 dB or more is about 746 MHz to about750.5 MHz, which is about 4.5 MHz, for example. Therefore, compared withthe above-described comparative example, it is clear that the frequencyrange across which attenuation is capable of being increased is widened.

FIG. 10 illustrates the relationship between the frequency range acrosswhich attenuation in the vicinity of the attenuation band Fx in theabove-described first preferred embodiment is improved by about 3 dB ormore over the case that is not the present preferred embodiment and thedistance between the IDTs when the distance between the IDTs is changed.

From FIG. 10, it is clear that, when the distance between the IDTs islarger than about 12λ, the frequency band across which attenuation canbe improved by about 3 dB or more over the case that is not the presentpreferred embodiment is narrow at around 2 MHz and substantially doesnot change, for example. In addition, when the distance between IDTs isabout 12λ or less, the frequency band across which an improvement ofabout 3 dB or more is capable of being achieved is made wider than about2 MHz. Therefore, it is clear that frequency band across which animprovement is capable of being made is made wider as the distancebetween IDTs is reduced.

As is clear from FIG. 10, when the distance between IDTs is about 12λ orless, according to a preferred embodiment of the present invention, thefrequency range across which attenuation is capable of being increasedis effectively widened. In addition, it is clear that, when theinter-IDT distance is preferably about 6λ or less, a frequency rangeacross which attenuation is capable of being increased up to at leastabout 4 MHz is secured, and the frequency range across which attenuationis capable of being increased is more effectively widened. In the stepof forming the IDTs on the piezoelectric substrate, the distance betweenthe IDTs is preferably about 0.25λ, or more in order to preventinterference between the IDTs.

In a second preferred embodiment of the present invention, a surfaceacoustic wave filter 21 illustrated in FIG. 11 is preferably usedinstead of the surface acoustic wave filter 11 illustrated in FIG. 1B.The surface acoustic wave filter 21 is a transversal surface acousticwave filter including IDTs having a slanted finger design.

As illustrated in FIG. 11, the surface acoustic wave filter 21 includesa piezoelectric substrate 22. A first IDT 23 and a second IDT 24 areprovided on the piezoelectric substrate 22. The plurality of electrodefingers of the first IDT 23 extend in a slanted direction thatintersects a direction perpendicular or substantially perpendicular tothe propagation direction of the surface acoustic waves. The second IDT24 is preferably configured in the same manner. An electrode fingerinterval of the first IDT 23 and the second IDT 24 from one side to theother side in the width direction of the piezoelectric substrate 22 isgradually changed. The passband is widened with such a slanted-fingersurface acoustic wave filter 21.

In FIG. 12 and FIG. 13, the solid lines respectively illustrate theamplitude characteristic and the phase characteristic of the surfaceacoustic wave filter 11 used in the first preferred embodiment and thealternate long and short dash lines respectively illustrate an amplitudecharacteristic and a phase characteristic of the slanted-finger surfaceacoustic wave filter 21. As is clear from FIG. 12 and FIG. 13, in theslanted-finger surface acoustic wave filter 21, the change in phase andthe change in amplitude become smaller with a change in frequency.Therefore, by using the slanted-finger surface acoustic wave filter 21,the phase of the band pass filter 10 and a signal of the surfaceacoustic wave filter 21, which defines and serves as a delay element,are maintained in a state of having close to opposite phases and thesame amplitudes across a wider frequency range. Therefore, the frequencyrange across which attenuation is capable of being increased is widenedeven more. A duplexer of the second preferred embodiment that uses thesurface acoustic wave filter 21 was provided and its frequencycharacteristics were evaluated.

The configuration of the duplexer was the same as that of the firstpreferred embodiment except that the surface acoustic wave filter 21 wasused. The specifications of the surface acoustic wave filter 21 were asfollows.

Number of pairs of electrode fingers of first IDT 23=5, number of pairsof electrode fingers of second IDT 24=15. Electrode finger intersectingwidth=28 μm. Wavelength λ, which is the period of electrode fingers,=1.51 μm to 1.64 μm and IDT separation=1.3λ.

The slanted-finger transversal surface acoustic wave filter 21 wasconnected in parallel with a band pass filter 10 having a transmissionfrequency of around 777 MHz to 787 MHz and a ladder circuitconfiguration.

The attenuation-frequency characteristic of the thus-configured filterdevice is represented by the alternate long and short dash line in FIG.14. In addition, for comparison, an attenuation-frequency characteristicof the band pass filter when not connected to the surface acoustic wavefilter 21 is represented by the solid line in FIG. 14. In addition, animportant portion of FIG. 14 is illustrated in an enlarged manner inFIG. 15.

As is clear from FIG. 14 and FIG. 15, according to this preferredembodiment, attenuation is increased in the vicinity of about 2.5 GHz,which is a frequency range at a frequency higher than about 1200 MHz,which is the transmission frequency. More specifically, attenuation inthe vicinity of about 2.5 GHz was around 53 dB in the case where thesurface acoustic wave filter 21 was not connected, whereas it waspossible to increase the attenuation to around 60 dB by connecting thesurface acoustic wave filter 21, for example.

In addition to the attenuation-frequency characteristics of the filterdevice of the second preferred embodiment of the present invention,attenuation-frequency characteristics of filter devices of third andfourth preferred embodiments of the present invention are illustrated inFIGS. 16 and 17. The third and fourth preferred embodiments of thepresent invention use the surface acoustic wave filter 11, which doesnot have a slanted finger structure, similarly to the first preferredembodiment of the present invention. In the third and fourth preferredembodiments of the present invention, the distances between the IDTs inthe surface acoustic wave filters are respectively about 1.3λ and about10.8λ, for example. In other respects, the configurations are preferablythe same or substantially the same as that of the elastic wave filter ofthe second preferred embodiment of the present invention.

As is clear from FIG. 16 and FIG. 17, the maximum value of attenuationin the vicinity of about 2.5 GHz is increased in the third and fourthpreferred embodiments compared with the second preferred embodiment ofthe present invention. However, it is clear that the frequency rangeacross which attenuation is increased is wider in the second preferredembodiment of the present invention. That is, as illustrated in FIG. 15,in the second preferred embodiment of the present invention, thefrequency range across which attenuation is capable of being increasedover the attenuation in the vicinity of about 2.5 GHz in the case wherethere is only the band pass filter 10 is about 95 MHz, for example. Incontrast, in the third and fourth preferred embodiments of the presentinvention, the frequency ranges across which the attenuation is capableof being increased over the attenuation in the vicinity of about 2.5 GHzin the frequency characteristic of just the band pass filter 10 areabout 45 MHz and about 25 MHz, respectively, for example. Therefore,according to the second preferred embodiment of the present invention,it is clear that the frequency range across which attenuation at adesired frequency outside of the band of the band pass filter 10 iscapable of being increased is widened even more. This is because, asdescribed above, the changes in amplitude and phase with changes infrequency are small in the slanted-finger surface acoustic wave filter21.

In the above-described first to fourth preferred embodiments of thepresent invention, a single surface acoustic wave filter 11 is connectedto the band pass filter 10. In various preferred embodiments of thepresent invention, a plurality of delay elements may be connected inparallel with the band pass filter 10. That is, as illustrated by thebroken line in FIG. 1, a second transversal surface acoustic wave filter11A may be connected in parallel with the band pass filter 10. In thiscase, it is preferable that the electrode finger pitch of the surfaceacoustic wave filter 11 and the electrode finger pitch of the surfaceacoustic wave filter 11A be different from each other. In this way,attenuation is increased in a plurality of frequency bands.

In the above-described preferred embodiments of the present invention, asurface acoustic wave filter is preferably included as the elastic wavefilter that defines the delay element, but a transversal boundaryacoustic wave filter may be used instead. A filter on the input or theoutput or both sides may include another element such as an inductor anda capacitor.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

What is claimed is:
 1. A duplexer comprising: a reception filter; and atransmission filter that includes a ladder filter including a pluralityof series-arm resonators and a plurality of parallel-arm resonators, anda first acoustic wave device that is connected in parallel with theladder filter; wherein the first acoustic wave device includes apiezoelectric substrate and a first interdigital transducer (IDT) and asecond IDT provided above the piezoelectric substrate, and a distancebetween the first IDT and the second IDT in an acoustic wave propagationdirection is about 12λ or less when a wavelength determined by anelectrode finger period of one of the first and second IDTs is denotedby λ.
 2. The duplexer according to claim 1, further comprising: atransmission terminal; and an antenna terminal; wherein the ladderfilter is connected between the transmission terminal and the antennaterminal.
 3. The duplexer according to claim 1, wherein the distancebetween the first IDT and the second IDT is about 6λ or less.
 4. Theduplexer according to claim 1, wherein the distance between the firstIDT and the second IDT is about 0.5λ.
 5. The duplexer according to claim1, wherein the transmission filter includes a second acoustic wavedevice that is connected in parallel with the ladder filter.
 6. Theduplexer according to claim 5, wherein an electrode finger pitch of thefirst acoustic wave device and an electrode finger pitch of the secondacoustic wave device are different from each other.
 7. The duplexeraccording to claim 1, wherein the reception filter includes alongitudinally coupled resonator elastic wave filter.
 8. The duplexeraccording to claim 1, wherein the reception filter includes a band passfilter.
 9. The duplexer according to claim 1, further comprising: acommon terminal connected to the transmission filter and the receptionfilter; and a matching inductor connected between the common terminaland a ground potential.
 10. The duplexer according to claim 1, whereinthe ladder filter is a band pass filter.
 11. The duplexer according toclaim 1, wherein the first acoustic wave device includes a surfaceacoustic wave filter.